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Japan.Steel plates with heavy gauge, which are mainly applied to large steel structures, are consequently subject to improved low-temperature toughness as well as weldability by the application of general TMCPs, The advanced TMCP has been developed so as to industrially manufacture SUF (

A dispersion of precipitates introduces a threshold stress which opposes deformation. The effect of precipitate shape on dispersion strengthening at elevated temperature has been examined. Three kinds of Mod.9Cr-1 Mo steels without Nb(C.N), with spherical Nb(C.N) and with complex Nb(C.N)+V(N.C) "V-wing" were designed to havethe same microstructures except for the shapeof precipitates. Thethreshold stresses were measuredby tensile and creep tests at 600'C. Two sorts of effects have been observed. The first is that complex precipitates restrain dislocation from climbing even in a condition in which dislocation climbs over a spherical precipitate. Mobile dislocations are considerably trapped at the concavity of the V-wings during deformation. The second effect is that the probability of dislocation trapping at V-wing is larger than that at spherical Nb(C. N). Thesetrapping effects play important roles in the strengthening effect of precipitate.

SynopsisTemper embrittlement and hydrogen embrittlement have been studied through systematic changes in composition of HY 130 steel using Charpy V-notch impact specimens and modified wedge-opening loaded specimens aged at 480 °C for 0 to 1 000 h. The results have shown that the threshold stress intensity for hydrogen-induced cracking decreases and the fracture appearance transition temperature increases as the impurity concentration increases at the grain boundaries with ageing time, and that an addition of 0.30 % AI and an elimination of Si, Mn, N and P in the HY 130 steel results in almost complete resistance to temper embrittlement and to hydrogen embrittlement. I. IntroductionBrittle fracture of steel can be induced by the segregation of impurities or hydrogen. Impurities segregate to grain boundaries during heat treatment or high temperature service; on the other hand, absorbed hydrogen segi egates to regions of high lattice expansion due to an applied tensile stress.Porter et al.'°~ studied the temper embrittlement of HY 130 steel due to a stress relief treatment and showed the embrittling effects of Mn and P; however, they could not produce as great an improvement in the resistance to embrittlement in production heats of HY 130 steel as that predicted by the statistical analysis of the laboratory heats due to the existence of excess free N.Cabral et al.7~ first showed that the threshold stress of a Ni-Cr steel in an H2SO4 solution decreased significantly when a tempered steel was aged so as to render it temper-embrittled.Later, Yoshino and McMahon8~ showed that temper embrittlement of a production heat of HY 130 steel (by step cooling) lowered drastically the threshold stress intensity for crack growth in an H2SO4 solution and changed the mode of cracking from mostly transgranular to completely intergranular. They interpreted their results due to an interaction between hydrogen embrittlement and impurity segregation at grain boundaries.Later, modified wedge-opening loaded specimens of the same heat of HY 130 steel were tested at fixed displacement in hydrogen gas by Briant et al.s~ The gaseous environment was used instead of an aqueous solution to avoid uncertainties about surface-controlled reactions and the influence of variations in the electrochemical conditions within a crack. They studied the effect of temper embrittlement and hydrogen cracking behavior with various grain boundary impurity concentrations by isothermal ageing at 480 °C for 0 to 1 000 h .